"As a nation, our job is to break our addiction to fossil fuels, not increase our dependence on oil," Sanders said in the statement. "I join with the Standing Rock Sioux Tribe and the many tribal nations fighting this dangerous pipeline.”

During the rally, Chairman Harold Frazier of the Cheyenne River Sioux Tribe said he and fellow council members spent some time on Capitol Hill in an attempt to drum up more support. And award-winning actress and activist Susan Sarandon -- one of Sanders's most outspoken allies -- is calling on the public to lobby Congress to stop the project.

Public pressure could play a role even as the lawsuit proceeds because final approval has yet to secured for drilling work at Lake Oahe. During the court hearing on Wednesday, Matt Marinelli, an attorney for the U.S. Army Corps of Engineers, the defendant in the case, said the process will trigger a 14-day notification window to Congress.

That statement seemed to catch William J. Leone, an attorney for the Dakota Access partnership, off guard. Up until that point, he said he thought his clients had all the necessary approvals to complete construction on the pipeline.

A drum group sings at the #NoDAPL rally outside the federal courthouse in Washington, D.C. Photo by Indianz.Com

Still, the notification process could be seen as a mere formality since the Army Corp has all but cleared the way for construction at Lake Oahe and that's one of the reasons the tribe is in court.

"There's been no invitation ... there's been no opportunity" for the tribe to participate in addressing its concerns about Lake Oahe,
Jan Hasselman, an attorney with Earthjustice, a non-profit firm that filed the lawsuit, said in court on Wednesday.

The tribe asked for a preliminary
injunction to stop the pipeline but Judge James
E. Boasberg declined to rule immediately, citing the complex nature of the dispute. He also said the lack of work at Lake Oahe gave him some reassurances as he prepares a decision, which he promised within two weeks.

While Sanders did not win the Democratic nomination for president, his tribal allies secured a provision in the party platform that calls on the federal government to ensure that its actions do not contribute to climate change. The language was proposed by Deborah
Parker, the former vice chair of the Tulalip Tribes in
Washington.